It will dawn on us when she appears (6)
I believe the answer is:
aurora
'it will dawn on us' is the definition.
The definition suggests a singular noun which matches the answer.
'she appears' is the wordplay.
I cannot really see how this works, but
'she' could be 'rora' and 'rora' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Other definitions for aurora that I've seen before include "goddess appears" , "Roman goodess of the dawn" , "skylights?" , "Atmospheric luminance" , "Ship that turned red" .)